Japanese Patent Application Publication No. 2011-34713 (hereinafter referred to as a “Document 1”) discloses a lighting fixture equipped with a visible-light-communication function that transmits a communication signal by using illumination light. The lighting fixture includes: a light source substrate on which a light source is mounted; a lighting circuit substrate which is electrically connected with the light source substrate and performs lighting control of the light source; and a visible-light-communication controlling substrate which superposes a communication signal on outgoing light from the light source. In the lighting fixture, the visible-light-communication controlling substrate is detachably disposed between the lighting circuit substrate and the light source substrate, and a design thereof can be accordingly made common between a fixture with a visible-light-communication function and a fixture with no visible-light-communication function. Control electrical power of the visible-light-communication controlling substrate is supplied from an output terminal of the lighting circuit substrate.
However, when a switch device for modulation in the visible-light-communication controlling substrate is turned off, a load voltage onto light-emitting devices (LEDs) in the light source is shut off, and accordingly the lighting fixture of Document 1 is put in a condition without substantial load with respect to the light source. If the light source is put in the unloaded condition, a voltage across the output terminal of a power supply unit in the lighting circuit substrate is likely to rise sharply. That is, a PFC circuit having a boost function for improving distortion of an input electric current is used for a general power supply unit of which electric power supply is commercial power AC, and accordingly if the light source is put in the unloaded condition, a voltage across the output terminal of the power supply unit rises close to output voltage of the PFC circuit. If the voltage across the output terminal of the power supply unit rises, an input voltage onto a control power supply of the visible-light-communication controlling substrate rises as well, which causes loss of circuit. In order to suppress the loss of circuit, it is necessary to mount high-priced parts having high pressure resistance or many parts on the power supply unit and the control power supply, which causes high production costs. Mounting the parts also requires a complicated circuit design, and accordingly the lighting circuit becomes bigger, thereby causing difficulty in miniaturizing the lighting fixture.